The present invention relates to methods and compositions for treating subterranean formations, and more specifically, to improved methods and compositions for generating acids for use downhole, for example, to at least partially degrade the acid-soluble portions of filter cakes deposited in subterranean formations.
Filter cakes, e.g., residues deposited on permeable media when servicing fluids contact those media under pressure, are formed in a variety of subterranean operations such as drilling, fracturing, and gravel packing. A filter cake is often desirable, at least temporarily, as it may prevent a fluid from flowing from a desired location to the surrounding subterranean formation. Moreover, the presence of a filter cake may add strength and stability to the formation surfaces on which the filter cake forms.
Filter cakes can form in many subterranean processes. For example, a drill-in fluid in an open hole can form a filter cake. Such filter cakes generally comprise an acid-soluble portion and a polymeric portion. Filter cakes also may be formed during a fracturing operation. As a fracture is created, a portion of the fluid contained in the viscous fracturing fluid may create a filter cake comprising deposited viscosifier and/or fluid loss control additives, inter alia, on the walls of the fracture and/or the formation.
No matter which process forms a filter cake, the filter cake generally acts as a physical barrier to fluid flow that may reduce fluid loss into the producing zone. The filter cake also presents a barrier to flow of liquid from the zone; thus, at some point before the well is put into production, the filter cake generally is removed so that the formation may be placed on production.
While filter cakes may be beneficial, it is generally necessary to remove filter cakes from producing zones once the well is placed into production. For example, to degrade the acid-soluble portion of a drill-in fluid filter cake, a conventional delayed-release acid system usually may be used. A common type of delayed-release acid system comprises esters that slowly hydrolyze to form acids that may ultimately degrade the acid-soluble portion of the filter cake. These delayed-release acid systems, however, can be problematic if they degrade the acid-soluble component of the filter cake too slowly or too quickly. Removal of only 1% to 2% of the bridging solids in the filter cake can result in a significant loss of fluid to the surrounding formation. If a delayed-release acid system is designed not to dissolve more than 1% or 2% of the acid-soluble portion of the filter cake in a chosen period of time (e.g., a 12-hour period), then total removal may take days, if not weeks. This is undesirable. On the other hand, if a delayed-release acid system is designed to totally degrade the acid-soluble portion within an acceptable “total cleanup time” (e.g., 24 to 48 hours), it is likely to cause hole instability and potential fluid loss problems during gravel pack placement. To control such fast-acting delayed-release acid systems, buffers (which are mixtures of weak acids and their conjugate bases) may be considered to achieve a delayed interaction of the acid with the acid-soluble portion of the filter cake for a desired time period. However, such conventional buffer systems have met with little success when used with these delayed-release acid systems, inter alia, because the esters may undergo acid- or base-catalyzed hydrolysis at pHs much below or above 7. Also, conventional buffers may suffer when exposed to components, such as calcium carbonate, in the filter cake and, as a result, the acid component of the buffer may be quickly consumed.